Symmetries in Quantum Mechanics Summer School on Quantum Mechanics (July 29 - August 2, 2019) Islamic University of Science and Technology, Awantipora, Pulwama, Jammu & Kashmir Govind S. Krishnaswami, Chennai Mathematical Institute www.cmi.ac.in/∼govind/teaching/awantipora-qm-sch19 [email protected], August 11, 2019 Contents 1 References 1 2 Generators of transformations, symmetries and conservation laws 2 2.1 Generators of space translations...................................................2 2.2 Generator of time translation and time evolution operator.....................................2 2.3 Angular momentum as the generator of rotations..........................................3 2.4 Example of rotations generated by orbital and spin angular momentum..............................3 2.5 Symmetries and conservation laws..................................................5 3 Parity: A discrete symmetry 6 4 Symmetry among identical particles: bosons & fermions 7 4.1 Identical and indistinguishable particles...............................................7 4.2 Two spin half electrons in Helium.................................................. 11 4.3 Coulomb and exchange interactions.................................................. 12 4.4 Electronic configuration of light elements and periodic table.................................... 13 5 Time reversal 14 5.1 Time reversal in classical mechanics................................................. 14 5.2 Time reversal in quantum mechanics................................................. 15 1 References 1. E Wichmann, Quantum Physics, Berkeley Physics Course, Vol 4. 2. R P Feynman, R B Leighton and M Sands, Feynman Lectures on Physics, Vol 3. 3. H S Mani and G K Mehta, Introduction to Modern Physics. 4. B Dutta-Roy, Elements of Quantum Mechanics. 5. J J Sakurai and J J Napolitano, Modern Quantum Mechanics, 2nd Ed. 6. L I Schiff, Quantum Mechanics, 3rd Ed. McGraw-Hill (1968). 7. P J E Peebles, Quantum Mechanics, Princeton Univ Press (1992). 8. B H Bransden and C J Joachain, Quantum Mechanics, 2nd Ed., Pearson (2000). 9. D J Griffiths, Introduction to Quantum Mechancis. 10. N Zettili, Quantum Mechanics: Concepts and Applications, Wiley 2nd Ed.,(2009) 11. R L Liboff, Introductory Quantum Mechancis. 12. R Shankar, Principles of Quantum Mechanics, 2nd Ed. Plenum Press (1994). 13. P M Mathews and K Venkatesan, A textbook of quantum mechanics, Tata McGraw-Hill (1977). 14. P A M Dirac, The principles of quantum mechanics, 4th Ed., Oxford (1958). 15. W Heitler, Elementary wave mechanics with applications to Quantum Chemistry. 16. L D Landau and E M Lifshitz, Quantum mechanics: Non-relativistic theory, 3rd Edition. 17. K Gottfried and T-M Yan, Quantum Mechanics: Fundamentals, 2nd Ed Springer (2003). 1 2 Generators of transformations, symmetries and conservation laws 2.1 Generators of space translations • What does it mean to say that momentump ^ generates translations, angular momentum generates rotations and the hamiltonian generates time translations? • Consider first a system with one degree of freedom so that the wave function or potential is @ a function of one variable f(x). Recall thatp ^ = −i~ @x . Under a translation x 7! x + δa, the function at the translated point is 0 1 2 00 1 @ ip^ δa f(x+δa) = f(x)+δa f (x)+ (δa) f (x)+··· = f(x)−δa (−i~) f(x)+··· = I + + ··· f: 2 i~ @x ~ It is in this sense that momentum generates an infinitesimal translation. More generally, for a finite translation one gets f(x + a) = eipa=~f(x), as we will see below. Physically, f could be a wave function or a potential. If pf = 0 we say that f is translation invariant since f(x + δx) = f(x). For example, a constant potential V (x) = V0 is translation invariant. Similarly, a potential V (x; y) is translation invariant in the z -direction. We say it is annihilated by pz , since pzV (x; y) = 0. • In 3d, under an infinitesimal translation r ! r + δa, a function changes to f(r + δa). Using the Taylor expansion 1 f(r + δa) = f(r) + δa · rf(~r) + δa δa @ @ f(r) + ··· (1) 2 i j i j We can write this in terms of momentum i f(r + δa) ≈ (1 + δa · p^)f(r) (2) ~ And for a finite translation by a = n δa we apply the infinitesimal translation n times and let n ! 1 holding δa fixed n i 1 i a·p^ f(r + a) = lim I + a · p^ f(r) = e ~ f(x): (3) n!1 ~ n We say that p^ is the infinitesimal generator of translations. If we expand the exponential we will recover the Taylor expansion. 2.2 Generator of time translation and time evolution operator • To find the generator of time translations, we observe that the Schr¨odingerequation i~ _ = H may be solved for short times to write iHδt iHδt (t + δt) ≈ (t) − (t) ) (t + δt) = 1 − (t) (4) ~ ~ In this sense, the hamiltonian generates infinitesimal time-translations of the wave function. As above, a finite time-translation is given by composing a succession of infinitesimal time- evolutions j (t)i = e−iHt=^ ~j (0)i. The operator U(t; 0) = e−iHt=~ is called the time evolution 2 operator. It is unitary if the hamiltonian is hermitian, since then U yU = eiHt=~e−iHt=~ = I . We may write the time evolution operator as an infinite exponential series iHt −it2 1 −it3 1 U(t; 0) = I − + H2 + H3 + ::: (5) ~ ~ 2! ~ 3! • Though U is not an observable, it is still an important operator in quantum mechanics. It is easy to find its eigenstates and eigenvalues. Suppose the eigenstates of the hamiltonian are Hj ni = Enj ni. Then one finds that the energy eigenstates are also eigenstates of the time evolution operator and that −iEnt=~ Uj ni = e j ni: (6) The eigenvalues of U are complex numbers of unit magnitude e−iEnt=~ . The eigenvalues of the −iEnt= time evolution operator give the time evolution of stationary states Ψn(t) = ne ~ . 2.3 Angular momentum as the generator of rotations • The small change in ~r upon making a rotation by small angle δφ counter clockwise about the axis δφ~ is δ~r = δφ~ × ~r. f(~r + δφ~ × ~r) ≈ f(~r) + δφ~ × ~r · r~ f(~r) (7) Now (A × B) · C = A · (B × C) since both of them measure the volume of a parallelepiped defined by the vectors A; B; C ordered so that C has a positive component along A × B. Thus using p = −i~r, i i f(~r + δφ~ × ~r) ≈ f(~r) + δφ~ · (r × p) f(~r) = I + δφ~ · L~ f(~r): (8) ~ ~ So angular momentum generates rotations. For a finite rotation counter clockwise about axis φ~ by angle jφ~j we have i φ~·L~ f(~r) 7! e ~ f(~r): (9) Here L · φ = Lxφx + Lyφy + Lzφz . We say that Lx generates a rotation about the x axis, Ly generates a rotation about the y axis and Lz generates a counter clockwise rotation about the z axis. • Now if V (r) is a spherically symmetric (central) potential, then we know it is unaffected by rotations. It follows that LV (r) = 0. In other words, LiV (r) = 0, or the angular momentum 2 P 2 operators annihilate the potential. It follows that L = i Li also annihilates a spherically symmetric potential. Check that this implies [Li;V (r)] = 0 for each i and consequently that [L2;V (r)] = 0. This result will be used in studying motion in a central potential, where one 2 shows that L and (say) Lz commute with the hamiltonian (say of the hydrogen atom), and 2 that L , Lz and H are simultaneous observables. 2.4 Example of rotations generated by orbital and spin angular momentum • Orbital angular mometum: Recall that a finite counter-clockwise rotation by angle jαj iL~ ·~α about the axis ~α is effected by R~α = e ~ iL~ ·~α (~r + ~α × r) = e ~ (~r): (10) 3 Of course, rotations do not affect r, they only affect the angles θ; φ. So we will focus on the angles and consider a wave function (θ; φ). Any such wave function can be expanded as a linear P combination of spherical harmonics (θ; φ) = lm clm lm(θ; φ). The clm can be determined by using the orthogonality and normalization of Ylm . We have seen that the angular momentum operators L~ are each block diagonal in the spherical harmonic basis. This means Lx;Ly;Lz have zero matrix elements between spherical harmonics with different values of l hYlmjLijYl0m0 i / δll0 : (11) iL~ ·~α So under a rotation, e ~ (θ; φ), if the initial state was one with a definite value of l then the transformed wave function will also have the same value of l. • As an example, let us consider how a p-wave state (r; θ; φ) = aY11 +bY10 +cY1;−1 transforms under a counter-clockwise rotation by angle α about the z -axis. In the (Y11;Y10;Y1−1 ) basis Lz is diagonal with diagonal entries (~; 0; −~). Thus we have 0a1 0 aeiα 1 iLzα/~ Rαz^ (aY11 + bY10 + cY1;−1) = e @bA = @ b A c ce−iα iα −iα Rαz^Y11 = e Y11;Rαz^Y10 = Y10; and Rαz^Y1−1 = e Y1−1: (12) In particular Y10 is unaffected by a rotation about z -axis, which is not surprising as it was independent of the azimuthal angle φ to begin with. This is consistent with the coordinate space wavefunctions ±iφ Y10 / cos θ; Y1;±1 / sin θe (13) if we observe that such a rotation augments the azimuthal angle φ by α. In particular, we observe that a rotation by α = 2π brings us back to the initial state, as we would expect. • Remark: Rotation of a physical vector can be thought of in two ways. (1) A rotation of the corresponding position vector ~r holding the surroundings (axes) fixed. Or (2) a rotation (by the negative angle) of the ambient space (i.e., the axes) holding ~r fixed. In both points of view, a rotation by α = 2π about any axis acts as the identity transformation for any function (~r).